A variety of analytical systems are used in the detection of specific constituents within a liquid sample. For example, a flow injection analysis system may be used to measure a specific analyte dissolved in water. In any analytical system, however, a sensor typically is used to measure or detect the specific constituent of interest.
A typical sensor may be in the form of a probe that acts as a transducer able to convert the presence or concentration of an analyte in the liquid sample to data of use to an analyst. Commonly, sensors that do not dissolve in the sample are referred to as electrodes. A variety of electrode sensors are used in the field of water analysis. In general, these electrodes are named after the analyte to which they respond. For example, electrode sensors that respond to ions in solution are called ion-selective electrodes. Further, they may respond only to a specific analyte, such as fluoride or glucose. Also, if the sensor's mechanism of sensing depends on an enzyme immobilized onto the surface of the electrode, the sensor is referred to as an enzyme electrode or a biosensor. Such biosensors often sense as their analytes, the "substrate" or analyte compound whose reaction with various reactants is catalyzed by the enzyme.
The above-described sensors typically are designed with an analyte-sensitive surface on one exposed end of the sensor. This surface may then be placed in contact with a potential, desired constituent, such as an analyte, in the liquid sample in various ways. For example, the sensor may be dipped into a sample solution or the sensor may be exposed to a flowing stream. In either case, the electrode is calibrated to output a response, e.g. a current or a voltage, proportional to change in concentration of the constituent of interest in the sample solution.
Preferably, the change in response or output of the electrode is due only to changes in the analyte concentration within the sample. When a change in the output signal is caused by something other than a corresponding change in concentration of the specific constituent of interest, the change results in an error signal.
Generally, the error signal may rise from a change in the sensitivity of the electrode or from an interference. For example, changes in the total ionic strength and/or temperature of the analyte solution may affect the sensitivity of the sensor. However, pressure changes, interfering compounds, gas bubbles, static electrical discharges, and changes in the resistivity of the electrode can provide an interference that leads to an error signal. One of the most difficult error signals to control is that which results from pressure changes acting on the liquid sample. Water solutions, for example, are incompressible and instantly transmit forces and pressures to all surfaces exposed to the solution, including the exposed sensor surface.
When sensing the presence of a specific constituent within a static liquid sample, the sensor is held steady while submerged in the liquid sample until a sufficient decrease occurs in the pressure related error signal. However, such batch measurements are inefficient due to the time required in waiting for a sufficient decrease in the pressure induced error signal. Generally, it is more efficient to test a liquid sample by flowing the liquid sample in a stream past the sensitive surface of the sensor. This may be accomplished by mounting the sensor in a flow cell.
A conventional flow cell consists of a sample stream inlet that opens into a chamber in which one wall is defined by the sensitive surface of the sensor. The liquid stream continually flows past the sensor to a waste outlet and on to waste. With this system, multiple samples can be pumped in series through the flow cell and past the sensor without movement of the sensor from one sample solution to another.
Because any perturbations to the flowing liquid stream can create pressure changes that cause the sensor to create error signals, the design of the flow path is important for accurate analysis of the liquid stream. Conventional flow cells do little to facilitate consistent, accurate sensor output by eliminating the potential for pressure changes in the liquid stream. It would be advantageous, for example, to eliminate sources of change in backpressure that result from perturbation of the liquid stream downstream from the sensor.